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Issue 10, 2008
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The observed and energetically feasible crystal structures of 5-substituted uracils

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Abstract

A search of the Cambridge Structural Database for crystal structures of 5-substituted uracils shows that, although there is a recurrent motif with symmetric hydrogen bonding and interdigitation of the 5-substituent R, a range of other hydrogen bonded ribbons, sheets and three-dimensional motifs are possible. In order to try and rationalize this, we have performed a combination of experimental studies and computational searches for low energy structures for the 12 simple 5-substituted uracils with R = H, CH3, CH2CH3, CHCH2, CN, OH, NH2, NO2, F, Cl, Br and I. Crystallization experiments on these compounds yielded the first single crystal X-ray determinations of 5-ethyluracil and 5-cyanouracil, as well as low temperature redeterminations of the disordered structures of 5-chlorouracil and 5-bromouracil. The lattice energies were calculated for the known crystal structures and compared with the computed lattice energy landscape for each molecule (except R = Br and I). Although the symmetric ribbon motif often dominates the computed crystal energy landscape, all of the molecules show a variety of different hydrogen bonding structures within a small energy range (5 kJ mol−1) of the global minimum and exhibit quite a diverse range of energetically competitive motifs. Thus, the range of crystallization outcomes, from polymorphism and other multiple forms, to the difficulty in growing single crystals (R = CHCH2 and NH2) probably reflects the sensitivity of the various hydrogen bonding motifs to the substituent and limited range of crystallization conditions that can be applied.

Graphical abstract: The observed and energetically feasible crystal structures of 5-substituted uracils

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Publication details

The article was received on 22 Apr 2008, accepted on 30 Jun 2008 and first published on 12 Aug 2008


Article type: Paper
DOI: 10.1039/B806763E
Citation: New J. Chem., 2008,32, 1761-1775
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    The observed and energetically feasible crystal structures of 5-substituted uracils

    S. A. Barnett, A. T. Hulme, N. Issa, T. C. Lewis, L. S. Price, D. A. Tocher and S. L. Price, New J. Chem., 2008, 32, 1761
    DOI: 10.1039/B806763E

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